Let’s stick with modern era.
OK so we are clear, we are no longer talking about the best ever tennis player.
Who is the best modern era player across all surfaces ?
It's generally considered that Ivan Lendl was the most consistent modern player across all surfaces. This is because he consistently preformed well across all surfaces at all times.
Borg has to be considered close to Lendl as the best across all surfaces.
Federer was dominant over hard court and grass.
Samprass was the king of grass.
Djokovic is the king of the hard court. Jimmy Connors says hi. I would have loved to see them two play each other.
Nadal is the king of clay.
McEnroe was the king of indoor.